[SY-I5] Dislocation-mediated boundary motion, dislocation-boundary interaction, and their effect on the mechanical behavior in fcc materials
Invited
In the current work, mechanisms for dislocation-mediated motion of and deformation at coherent boundaries in fcc materials is investigated along with the resulting mechanical behavior.
On the modeling side, molecular dynamics (MD) simulations are carried out on bulk single and bi-crystal Cu, the latter containing two Σ3(111) boundaries. These are subject to loading conditions varying between shear loading parallel to the boundaries and perpendicular uniaxial loading. In the case of pure shear loading, MD results demonstrate that <112> shearing on Σ3(111) results in monotonic flow depending on the shear sign, whereas <110> shearing leads to oscillatory flow. As it turns out, this difference in behavior can be related to corresponding changes in the coincidence site lattice (CSL). In particular, in the case of <112> shearing, new potential boundary positions in the CSL occur every three atomic layers. Depending on the shear sign, however, one of these positions is closer to the current boundary position than the other. Since less energy is required for the boundary to shift to the closer position, the boundary moves to this position. In contrast, <110> shearing results in new potential boundary positions in the CSL having the same distance to the current boundary. Consequently, none of these is favored energetically, resulting in oscillatory motion of the boundary between these.
On the experimental side, nano-indentation experiments with 2µm sized spherical indenter tips are used to study the impact of coherent Σ3(111) boundaries on the yield strength distribution in copper. The indents are performed either inside a grain or close to a coherent Σ3(111) boundary aligned normal to the sample surface. The force-displacement curves show elastic loading following Hertz´s predictions until a sudden displacement burst - a “pop-in” - is observed. The maximum shear stress beneath the indenter tip at the pop-in force is interpreted as yield stress. The statistical behavior of the yield stress is analyzed via cumulative probability (CP) plots. The CP plots show a significantly lower average yield stress at the boundary with respect to the single grains and an extremely narrow distribution at the boundary. The later finding suggests that the mechanism responsible for dislocation source activation is omnipresent at the boundaries.
On the modeling side, molecular dynamics (MD) simulations are carried out on bulk single and bi-crystal Cu, the latter containing two Σ3(111) boundaries. These are subject to loading conditions varying between shear loading parallel to the boundaries and perpendicular uniaxial loading. In the case of pure shear loading, MD results demonstrate that <112> shearing on Σ3(111) results in monotonic flow depending on the shear sign, whereas <110> shearing leads to oscillatory flow. As it turns out, this difference in behavior can be related to corresponding changes in the coincidence site lattice (CSL). In particular, in the case of <112> shearing, new potential boundary positions in the CSL occur every three atomic layers. Depending on the shear sign, however, one of these positions is closer to the current boundary position than the other. Since less energy is required for the boundary to shift to the closer position, the boundary moves to this position. In contrast, <110> shearing results in new potential boundary positions in the CSL having the same distance to the current boundary. Consequently, none of these is favored energetically, resulting in oscillatory motion of the boundary between these.
On the experimental side, nano-indentation experiments with 2µm sized spherical indenter tips are used to study the impact of coherent Σ3(111) boundaries on the yield strength distribution in copper. The indents are performed either inside a grain or close to a coherent Σ3(111) boundary aligned normal to the sample surface. The force-displacement curves show elastic loading following Hertz´s predictions until a sudden displacement burst - a “pop-in” - is observed. The maximum shear stress beneath the indenter tip at the pop-in force is interpreted as yield stress. The statistical behavior of the yield stress is analyzed via cumulative probability (CP) plots. The CP plots show a significantly lower average yield stress at the boundary with respect to the single grains and an extremely narrow distribution at the boundary. The later finding suggests that the mechanism responsible for dislocation source activation is omnipresent at the boundaries.